social insects.
But we have not as yet touched on the climax of the difficulty; namely,
the fact that the neuters of several ants differ, not only from the
fertile females and males, but from each other, sometimes to an almost
incredible degree, and are thus divided into two or even three castes.
The castes, moreover, do not generally graduate into each other, but
are perfectly well defined; being as distinct from each other, as are
any two species of the same genus, or rather as any two genera of the
same family. Thus in Eciton, there are working and soldier neuters,
with jaws and instincts extraordinarily different: in Cryptocerus, the
workers of one caste alone carry a wonderful sort of shield on their
heads, the use of which is quite unknown: in the Mexican Myrmecocystus,
the workers of one caste never leave the nest; they are fed by the
workers of another caste, and they have an enormously developed abdomen
which secretes a sort of honey, supplying the place of that excreted by
the aphides, or the domestic cattle as they may be called, which our
European ants guard or imprison.
It will indeed be thought that I have an overweening confidence in the
principle of natural selection, when I do not admit that such wonderful
and well-established facts at once annihilate my theory. In the simpler
case of neuter insects all of one caste or of the same kind, which have
been rendered by natural selection, as I believe to be quite possible,
different from the fertile males and females, in this case, we may
safely conclude from the analogy of ordinary variations, that each
successive, slight, profitable modification did not probably at first
appear in all the individual neuters in the same nest, but in a few
alone; and that by the long-continued selection of the fertile parents
which produced most neuters with the profitable modification, all the
neuters ultimately came to have the desired character. On this view we
ought occasionally to find neuter-insects of the same species, in the
same nest, presenting gradations of structure; and this we do find,
even often, considering how few neuter-insects out of Europe have been
carefully examined. Mr. F. Smith has shown how surprisingly the neuters
of several British ants differ from each other in size and sometimes in
colour; and that the extreme forms can sometimes be perfectly linked
together by individuals taken out of the same nest: I have myself
compared perfect gradations of this kind. It often happens that the
larger or the smaller sized workers are the most numerous; or that both
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